Contamination of particulate materials (e.g., soils, sediments, sludges, or muds) can arise from hazardous materials being spilled, leaked, discharged, co-processed, or buried at a site, or from intrusion of contaminants from offsite sources. As an example, leaking underground storage tanks have contaminated the soil at sites with petroleum hydrocarbons and lead, and caused similar contamination of adjacent sites through migration in the subsurface. In general, contaminants which may be found in particulate materials include liquids, which may have associated vapors, and solids. Contaminants may be physically or chemically attached to particles, or may be present as a separate phase between particles, such as non-aqueous phase liquids (NAPLs). Liquid contaminants include petroleum hydrocarbons, coal tars, and industrial solvents, while solid contaminants include salts, metals, and organic materials (e.g., explosives, pesticides).
Remediation of contaminated particulate materials implies the removal of contaminants and their impacts for the general protection of health and for the benefit of the environment. For example, remediation at former industrial (“brownfield”) sites may be a prerequisite for redevelopment for residential, commercial, or new industrial use. Remediation is generally subject to an array of regulatory requirements, and also can be based on assessments of human health and ecological risks in light of planned future use.
Site remediation methods addressing contamination can be classified in general terms as: (1) ex situ methods in which the particulate material is displaced and treated or disposed of in a waste facility; and (2) in situ methods in which the particulate material remains in place for treatment. Stabilization, solidification, or containment, while not remediation methods in themselves, may also be used to prevent the contamination from becoming more widespread. Treatment methods for organic contaminants are numerous and varied, with examples including: thermal approaches like incineration and thermal desorption; washing/flushing with aqueous solutions or organic solvents; bioremediation, in which microbial activity is stimulated to achieve enhanced biodegradation; and chemical oxidation, typically with an aqueous solution or gas. Combinations of methods are also common, such as washing with thermal treatment of separated, highly contaminated fine particles.
Of the thermal remediation methods for particulate materials, incineration is generally the most effective for destroying organic contaminants due to the high temperatures achieved. However, high temperatures also have greater associated fuel costs and tend to degrade native properties of soils and sediments. Due to the range of wastes fed into incinerators, their emissions must also be carefully monitored and controlled (e.g., through temperature and filtration). Thermal desorption, in which organic contaminants are desorbed from particulate materials and combusted in a burner, is a leading alternative to incineration. Thermal desorption is a cooler, less harsh treatment than incineration, but may be ineffective for particulate materials with high moisture contents or contaminant levels. While not as fuel-intensive as incineration, thermal desorption also has substantial fuel costs associated with both the desorbing and combusting of contaminants.
Remediation technologies are frequently benchmarked by cost to offsite disposal at a waste facility, which relocates contaminated particulate materials to an engineered site for long-term storage. Offsite disposal is often the most economical option for these materials and has a relatively low risk of failure in reaching regulatory criteria at a site. Except when transportation is impractical due to distance or quantity of material, few technologies can routinely compete with its combination of reliability and cost effectiveness. One of the most commonly employed alternatives to offsite disposal for organic contamination is bioremediation, which can cost roughly half the price. While bioremediation provides significant savings over offsite disposal, its application is generally restricted to particulate materials with relatively low levels of contamination, especially in the more refractory hydrocarbons. There is therefore a need for a cost-effective remediation technology that is effective when bioremediation cannot be applied.
Smoldering, which is a flameless combustion process, is a promising new approach for remediating particulate materials containing organic contamination. Smoldering may be sustained in a particulate material provided sufficient fuel is present. This process occurs naturally, for example, in underground peat fires. However, organic contaminants can also provide sufficient energy for self-sustaining smoldering combustion under the right conditions. Generally, these conditions include high enough contaminant concentrations, a supply of air, adequate retention of heat, and an initial source of heat to ignite the smoldering front. If these conditions are met, smoldering can be used as a process to remediate particulate materials, virtually eliminating all organic contaminants.
Smoldering combustion can be initiated in an in situ approach by actively heating a small region of contaminated particulate material below the surface and introducing air once that region has reached ignition temperature (typically 200-400° C.). The heater may then be deactivated, while the air supply is maintained to sustain a smoldering front, which propagates through the bed of particulate material destroying contaminants. Provided there is sufficient fuel for the process in the particulate material, smoldering can be self-sustaining in the sense that no further active heating is required after ignition, as the contaminants themselves supply the heat required for their ongoing destruction.
Smoldering has the potential to provide thorough contaminant removal in a cost-effective process. Unlike bioremediation, smoldering is capable of remediating particulate materials with high levels of organic contaminants, including the more refractory contaminants associated with events like crude oil spills. It is, moreover, facilitated by higher contaminant concentrations and is therefore naturally suited to heavily contaminated particulate materials. In addition, the cost of smoldering on a proportionate weight basis may be similar to bioremediation as a result of the savings on fuel costs, which are significant costs for other thermal remediation technologies (e.g., incineration and thermal desorption). However, like other forms of thermal remediation, smoldering thoroughly removes combustible contaminants, enabling stringent remediation standards to be met. Smoldering therefore offers the possibility of a thorough, robust, and cost-effective technology for remediating particulate materials contaminated with organic compounds.
Smoldering has been previously considered as a beneficial adjunct to thermal desorption. For example, it was recognized that hot gases used to desorb hydrocarbons could also combust molecules in the soil, as described in U.S. Pat. No. 5,193,934. In addition, the use of porous burners has been described in WO 95/3045 and WO 95/34349 to destroy desorbed hydrocarbons in a smoldering process with the heat of combustion recycled to assist in subsequent desorption. The use of smoldering as a primary strategy for soil remediation has been described by Gerhard et al. in Proc. Combust. Inst. 32, 1957 (2009), Environ Sci. Technol. 43, 5871 (2009), and Environ Sci. Technol. 45, 2980 (2011), as well as in situ methods described in CA 2 632 710 and US 2009/0180836, which have been the focus of reported field work to date. In certain respects, namely the use of subterranean combustion, in situ smoldering resembles the enhanced oil recovery method known as in situ combustion or fireflooding, described, for example, by Moore et al. in Fuel 74, 1169 (1995).
More recently, US Patent Application Publication No. 2012/0272878 to Grant et al. describes the application of smoldering for the volumetric reduction of organic liquids. US Patent Application Publication No. 2012/0288332 to Thomas et al. describes a method for remediating porous materials by fuel-assisted smoldering. While both disclose methods for enhancing smoldering with supplemental fuel sources, the focus is on fuel sources such as oily waste and petroleum hydrocarbons that are unlikely to confer environmental benefits to the smoldered products (e.g., treated soils and sludges), and which may, in some instances, be deleterious if complete destruction of the supplemental fuel source is not achieved or if more hazardous emissions are produced.